Hydraulic elevators are commonly used instead of traction type elevators in low rise applications. The advantage of the hydraulic elevator is its lower cost. This advantage, however, may be offset by the lack of precision control of the ride as compared to traction elevators.
In a typical hydraulic elevator, the flow of fluid to and from a hydraulic cylinder causes the elevator car to ascend and descend within the hoistway. During the ascent operation, the fluid is pumped from a tank by a pump and flows through a control valve before entering the cylinder. During the descent operation, the control valve opens to permit the fluid to flow from the cylinder and into the tank under the pressure of the car.
One type of hydraulic valve commonly used to transition between stopping and full speed includes a valve stem actuated by a solenoid to direct the flow of fluid through the valve. In this type of valve, in order to lower the elevator the solenoid is activated to permit a flow of fluid into a cylinder closed off by a piston. Upon sufficient pressurization of the cylinder, the piston will open to permit fluid flow through the valve and the car to descend. Balancing the fluid pressure on both sides of the valve prior to opening the valve provides a smooth and gentle start to the descent.
A second type of transition valve includes a valve stem actuated by an electric motor. In this type of valve, flow is controlled by a stepper motor that moves a flow control valve. The amount of flow is programmed and controlled through feedback to produce a desired velocity profile for the elevator car. The purpose of the motor actuated control valve is to produce more precise control of the motion of the elevator car and thereby a smoother, safer ride for the passengers that more closely approximates the ride of a traction type elevator. The main drawback to the motor actuated type of control valve is the additional complexity and cost associated with it.
The above art notwithstanding, scientists and engineers under the direction of Applicant's Assignee are working to develop hydraulic elevators that provide the desired control of the car motion without being prohibitively expensive.